One such ballast circuit is shown in U.K. Patent No. 2124042B. The circuits described in this patent are so called capacitive charge pump circuits including a reservoir capacitor connected across the outputs of a full wave rectifier which is in turn connected to an AC supply, the reservoir capacitor being shunted by a series arrangement of two switching devices. A discharge path is provided from the reservoir capacitor, through an output load comprising a series resonant circuit constituted by an inductor and a parallel arrangement of a discharge lamp and a resonating capacitor connected across the cathodes of the lamp, so as to periodically charge a control or charge pump capacitor, this lowering the load voltage and drawing current from the rectified supply. The reservoir capacitor is subsequently recharged by current flowing from the inductor at times defined by the alternate switching of the two switching devices. The circuit is arranged so that the voltage across the reservoir capacitor is always greater than the peak of the mains supply.
Thus in operation of this circuit current and energy can be taken from the mains at all parts of the mains cycle resulting in a low harmonic content waveform being drawn from the supply.
It will be seen that the effectiveness of such a charge pump circuit is dependent on the reservoir capacitor voltage, and the amount of circulating current in the parallel arrangement of the lamp and resonating capacitor. The amount of this circulating current is determined by the value of the resonating capacitor and the operating current of the lamp. As the resonating capacitor is connected across the lamp cathodes, it provides cathode heating current. Thus the value of the resonating capacitor is limited by the maximum current with which the cathode can be driven without long term damage by over heating, this causing a consequential limitation on the amount of circulating current possible, and thus the amount of charge which can be pumped.
It is possible to place an additional capacitor across the lamp thus providing a parallel current path to the cathode circuit in order to increase the circulating current without an accompanying increase in cathode current. Such an arrangement creates problems however in that in normal operation the switching devices will operate at a frequency higher than that of the output resonant circuit constituted by the inductor, lamp, resonating capacitor and additional capacitor. If the lamp is removed, or a cathode breaks during operation of the lamp, the remaining resonant circuit comprising the inductor and additional capacitor will have a higher resonant frequency than that of the original resonant circuit. Consequently the remaining resonant circuit may be instantaneously at or below resonant frequency. This situation may lead to damage to the switching devices due to over current or capacitive switching. Furthermore a large voltage may be left across the lamp terminals thus creating a safety hazard. It is also the case that without the additional capacitor the resonant circuit is broken if the lamp is removed or a cathode is broken; this safety feature is lost if an additional capacitor is used.